wasi: clean up core platform abstractions

[DePasqualeOrg]

This is the first PR split out from the larger WASI changes in #11712.

Changes

• Use rustix::fs::Stat for WASI FileInformation so same-file checks use device/inode identity instead of a file-type/size heuristic.
• Make read_fs_list() return UResult<Vec<MountInfo>> on WASI, matching other platforms while still returning an empty mount list.
• Use the WASI-specific OsStrExt/OsStringExt imports in env.
• Add inert WASI tail process-check stubs so the shared follow code compiles while --pid remains unsupported.
• Remove an unreachable nested cfg branch from non-WASI OwnedFileDescriptorOrHandle::into_stdio().
• Update WASI comm tests for corrected same-file detection.

[oech3]

closes #12427

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[sylvestre]

please refactor to avoid the allow dead code

[DePasqualeOrg]

Okay, I think we can just remove ProcessChecker for WASI. Since the original PR is from a couple months ago, I don't remember why exactly it was added.

[codspeed-hq]

Merging this PR will not alter performance

✅ 345 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing DePasqualeOrg:wasi-core-platform-abstractions (7db371a) with main (c477de7)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[oech3]

Is this ready to merge?

[DePasqualeOrg]

If's ready for whatever you want to do with it. I have coreutils working in Wasm and wanted to upstream my changes here in case it's useful to others. However, the process has dragged on for months, and there's been some unnecessary friction.

[DePasqualeOrg]

Now there's a new merge conflict. I've already resolved several other such conflicts in the past few months.

I think I will just leave #11712 as a reference for whoever is interested in Wasm support and maintain my own private patches, rather than trying to upstream changes here.